Conventional mobile communication circuits often include two stage and/or three stage amplifier circuits comprising a series of amplifiers. These may be constant gain or variable gain amplifiers. Constant gain amplifiers can be damaged under high supply voltage (e.g., high battery voltage) conditions that may occur as a result of supply current surges. For example, some cellular phones use batteries including three 1.5 volt cells in series, yielding a maximum theoretical voltage of 4.5 volts. After the cell phone is turned on, battery voltage (Vbat) may be 4.25 volts for a few seconds (for a short time at start up). Similarly, charging the battery during use may cause relatively high battery voltages. However, most of the time, the battery voltage is about 3.5 volts during operation. Thus, many circuits, such as constant gain amplifiers, are designed to operate efficiently at a nominal battery voltage (Vbatnom) of 3.5 volts. In these designs, a battery voltage in excess of Vbatnom may cause a maximum output power that exceeds physical limits (design limits) of a constant gain amplifier. If the constant gain amplifier is designed (tuned) for operating with a nominal battery voltage of 3.5 volts, then the constant gain amplifier may be damaged if a battery voltage of 4.25 volts is received. Further, the constant gain amplifiers can also be damaged by high input RF drive levels. The RF-based damage can be a function of both battery voltage and input RF drive levels.
Thus, amplifier topologies and techniques are needed to protect the constant gain amplifier from supply voltage conditions exceeding design limits.